Comparison of stepper motor and servo motor

Both stepper motors and servo motors can greatly improve the accuracy of the control loop. The stepper motor rotates at a fixed angle (step angle), and has no accumulated error, and has been widely used in various open loop controls. The servo motor can convert the received electrical signal into an angular displacement or angular velocity output on the motor shaft. It is characterized by no rotation when the signal voltage is zero, but its speed will decrease as the torque increases.

First, Working principle

1. The working principle of a stepper motor. A stepper motor is an actuator that converts electrical pulse signals into angular displacement. When the stepper driver of the motor receives the pulse signal, it will drive the stepper motor to rotate the step angle according to the set rotation direction. It rotates gradually at a fixed angle. In the control system, the angular displacement of the motor can be controlled by controlling the number of electric pulses, so as to achieve the purpose of precise positioning. At the same time, the motor can be controlled by the pulse frequency. Rotation of speed and acceleration, so as to achieve the purpose of speed regulation.

2. The working principle of the servo motor

The rotor inside the servo motor is a permanent magnet. The three-phase electricity controlled by the driver forms an electromagnetic field. The rotor of the servo motor rotates under the action of the electromagnetic field. At the same time, the motor feeds back the rotation signal to the driver through the encoder to adjust the rotation angle of the rotor. In the whole system, it can be said that the accuracy of the encoder determines the accuracy of the servo motor.

Second, the difference between stepper motor and servo motor

1. Difference of low frequency characteristics

Stepping motors are prone to low-frequency vibration at low frequencies. The vibration frequency is related to the load and drive performance. It is generally believed that the vibration frequency is half of the no-load take-off frequency of the motor. This kind of fixed frequency vibration phenomenon depends on the working principle of the stepper motor. This is very detrimental to the normal operation of the machine. When the stepper motor is running at low speed, half of them use damping technology to overcome low-frequency vibration, such as adding a damper to the motor or using subdivision technology on the drive. However, it is impossible to completely avoid this low-frequency vibration phenomenon in any case.

The AC servo motor runs very smoothly, and there is no low-frequency vibration even at low speeds. The AC servo system has resonance suppression characteristics, which can make up for the lack of mechanical rigidity, and the system has a frequency analysis function that can detect the resonance point of the machine and then adjust it. The system can reduce resonance.

The low frequency characteristics of servo motors are better than stepper motors.

2. Different control methods

The stepper motor controls the rotation angle of the motor by the number of pulses, one pulse corresponds to the stepping angle, and the servo motor controls the rotation angle of the motor by controlling the length of the pulse time.

3. Different torque frequency characteristics

The torque of a stepper motor decreases with the increase of the speed, and will drop sharply at a higher speed, so its maximum speed is generally 0~1500RPM. The AC servo motor has a constant torque output, that is, it can output a rated torque within its rated speed (usually 2000RPM~4000RPM), and output a constant power above the rated torque.

4. Different overload capacity

Stepper motors usually do not have overload capability. Due to the small overload capacity of the stepper motor, in order to overcome the moment of inertia, a stepper motor with a larger torque must be selected when selecting the model, and the machine does not require too much torque. Normally, a waste of torque will occur phenomenon. AC servo motors usually have a strong overload capacity.

Third, Attention points when choosing stepper motor and servo motor

1. The choice of stepper motor

1) Select holding torque: holding torque refers to the torque that the stator locks the rotor when the stepper motor is energized but not rotating. Since the torque of the stepper motor is close to the holding torque during low-speed operation, the torque of the stepper motor decreases rapidly as the speed increases, and the output power also changes as the speed increases. Therefore, the holding torque is a measure of one of the most important parameter steps of the motor's load capacity.

2) Choose the number of phases: Two-phase stepping motors have low cost, the step angle is usually 1.8 degrees, the vibration is larger at low speed, and the torque drops faster at high speed. It is suitable for high speed, low accuracy and stability requirements. The minimum step angle of a three-phase stepper motor is 1.5 degrees, the vibration is less than that of a two-phase stepper motor, the low-speed performance is better than that of a two-phase stepper motor, and the maximum speed is also higher than that of a two-phase stepper motor. Two-phase stepper motor, suitable for high-speed, high-precision and stable performance requirements. When choosing a stepper motor, you should follow the principle of selecting the motor first and then the driver. First determine the load characteristics, and then compare the static torque and torque-frequency curves of different types of stepper motors to find a matching motor. When the mechanical characteristics are high, a mechanical deceleration device should be used to make the motor work in a state with the highest efficiency and the lowest noise. If necessary, please solve the problem by changing the voltage, frequency or adding dampers to avoid making the motor work in the vibration area.

3) Select the driver and the number of subdivisions: For the stepper motor driver, it is best not to choose the full-step state, because the full-step state is easy to cause vibration, please try to choose a small current, large inductance and low voltage drive, and only use it when low vibration is required . For high-precision, only subdivision drives are equipped, and high-voltage drives are used in high-torque motors to obtain better high-speed performance. In the occasions with high speed, low accuracy and stability, it is not necessary to choose a high subdivision drive in order to save costs. In the case of extremely low speed, a larger number of subdivisions should be selected to ensure stable operation and reduce vibration and noise.

2. Precautions when choosing a servo motor

1) Load/motor inertia ratio: Correctly setting the inertia ratio parameter is the prerequisite for giving full play to the best performance of the mechanical and servo system. This is particularly prominent in systems that require high speed and high precision. There is a lot of work to be done for the high speed and inertia ratio of the servo system. When the load motor inertia ratio is too large, the adjustment range of the servo parameters becomes smaller. The higher the speed, the worse the vibration suppression ability and the more unstable the control. If no adaptive adjustment is made, the default adjustment parameter of the servo system is 1-3 times the inertia ratio of the load motor, and the system will reach the best working condition.

2) Selection of speed: First, the selection of the motor should be calculated according to the fast stroke speed of the mechanical system. The speed of the fast-stroke motor should be strictly controlled within the rated speed of the motor, and should be used within the range close to the rated speed of the motor in order to effectively utilize the power of the servo motor. For a servo motor, the rated speed, maximum speed and allowable instantaneous speed have the following relationship: allowable instantaneous speed>maximum speed>rated speed. When the servo motor is operating between the minimum speed and the rated speed, this is a constant torque speed regulation. When working between the rated speed and the maximum speed, it is constant power speed regulation; constant power speed regulation means that the output torque is large at low speed, and the output torque is small at high speed, that is, the output power is constant. Constant torque speed regulation means that the output torque of the motor at high speed and low speed is the same, that is, the output power at high speed is large, and the output power at low speed is small.

3) Short-time characteristics of the servo motor, namely acceleration and deceleration characteristics: In addition to the continuous area, the servo motor also has short-term operating characteristics, such as the acceleration and deceleration characteristics of the motor, which is expressed by the maximum torque; even if the capacity is the same, the maximum torque will be different.

The maximum torque will affect the acceleration time constant of the drive motor. According to the maximum torque of the motor, select the capacity of the motor.

By comparison, it can be found that the servo motor is superior to the stepper motor in many aspects, but the use of a stepper motor, the design of the control system is relatively simple and easy to implement, and the use of a servo motor, the control system cost is higher, and the control is more difficult.

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